In the graphic arts, it is desirable to produce a three or more color proof to assist in correcting a set of color separation films prior to using them to produce printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplication of the desired halftone image. Visual examination of a color proof should show the color rendition to be expected from press printing using the color separations and any defects on the separations which might need to be altered before making the printing plates.
Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires that all of the actual printing steps be performed. However, this conventional method of color proofing is costly and time consuming.
Photoimaging processes can also be used to produce color proofs. There are two general types of photoimaging methods; namely the overlay type and the single sheet type.
In an overlay type of color proofing method, an independent transparent plastic support is used for producing an image of each color separation film by applying a photosensitive solution of the corresponding color. A plurality of such supports carrying images of the corresponding colors are then superimposed upon each other over a white sheet to produce a color proofing composite. The primary advantage of the overlay method is that proofs can be made quickly and can serve as a progressive proof by combining any two or three colors in register. However, this type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet. As a result, the impression of the color proofing composite thus prepared is vastly different from that of copies actually obtained with conventional printing presses and with proof presses. Examples of such overlay approaches are contained in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films on a single receiver sheet. This is done by utilizing a single opaque support and by applying toners, transferring images, or processing solutions or coatings of photosensitive material of corresponding colors on the opaque support in succession. The single sheet method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system.
The present invention relates to improved negative- and positive-working proofing sheets of the solid layer transfer class. Color proofing films of this type are composed of a sheet substrate and at least a photosensitive member attached to the substrate. The photosensitive element is transferred to a receiver sheet by lamination generally with heat and pressure. After exposure, the transferred photosensitive element on the receiver sheet is developed preferably with an aqueous alkaline solution.
An example of a negative-working, solid transfer system is described in U.S. Pat. No. 4,650,738. The presensitized element comprises a substrate having a release surface, a pigmented photsensitive layer in contact with the release surface, and a heat sensitive adhesive layer in contact with the photosensitive layer. The element is laminated to a receiver sheet via the adhesive, the substrate peeled off, the photosensitive layer exposed, and the nonexposed areas removed during development with an aqueous, alkaline developer.
An example of a positive-working, solid transfer system is described in U.S. Pat. No. 4,659,642. The polymeric diazonium compound in the above negative-working element is replaced by a naphthoquinone diazide compound in the positive-working, presensitized element, while the other ingredients remain essentially the same. The positive-working element is processed in the manner as performed above.
In the solid transfer type of color proofing, the receiver sheet must undergo a high temperature and pressure lamination step. It must also undergo wet development. In a usual four color reproduction, the receiver sheet must undergo lamination and development repeatedly. Most printing papers cannot withstand these treatments and easily degrade. Therefore special, dimensionally stable and developer resistant receiver sheets, such as white opaque plastic films, are required. This is disadvantageous because the printer must judge the image quality on an artificial substrate rather than on the paper on which he ultimately intends to print. The present invention provides an improved method for producing an image on such a degradable receiver sheet whereby the desired receiver sheet is protected during the subsequent treatment steps. As a result virtually any paper stock can be used and the printer has an improved estimate of the quality of the image to be reproduced.